1. Field of the Invention
The present invention relates to a semiconductor device comprising a Schottky barrier diode wherein a PN junction is formed on a semiconductor layer located below a barrier metal film, and more particularly, to a semiconductor device having an intermediate semiconductor layer for pinching off a conductive path during the application of a reverse bias.
2. Description of the Related Art
Recently, most components of electronic apparatuses are ICs, and the power-supply devices incorporated in these apparatuses are generally of a switching regulator type, which are small and light and also operate with high efficiency. Most of the switching regulators have a Schottky Barrier Diode (SBD) rectifier used an output rectifying element. The SBD rectifier has a small forward voltage drop V.sub.F and a short reverse-recovery time. However, the SBJ rectifier now widely used is disadvantageous in two respects. First, a relatively large reverse leakage current often occurs, and second, the reverse breakdown voltage is low.
A new type of a SBD rectifier has been developed with a small reverse leakage current, and a high reverse breakdown voltage. (See Published Examined Japanese Patent Application No. 59-35183.) Such a rectifier is called a Junction Barrier-Controlled Rectifier Schottky (JBS). As is shown in FIG. 1, the substrate 10 of this JBS rectifier comprises an N.sup.+ -type silicon layer 12, an N.sup.- -type silicon layer 14 formed on the layer 12 by means of epitaxial growth. Metal electrode film 16 is formed on the lower surface of the layer 12. A plurality of P.sup.+ -type silicon areas 18 are formed in the upper surface of the layer 14. Oxide film 20, having an opening, is formed on the upper surface of the layer 14. Metal film 22, for example, chromium film, which forms a Schottky barrier, is formed partly on the exposed portion of the layer 14 and partly on the oxide film 20, as is illustrated in FIG. 1. The areas 18 are spaced apart from one another, by such a distance such that they are connected by an extending depletion layer 24 when a negative voltage is applied to the layer 16 and the film 22, respectively.
When a forward voltage is applied to the JBS rectifier shown in FIG. 1, most of a forward current flows through the Schottky barrier formed of the metal film 22 and the N.sup.- -type silicon layer 14, as is shown by arrows A. Hence, the forward voltage drop in this JBS rectifier is nearly equal to that in the ordinary Schottky diode which has no P.sup.+ -type silicon areas.
A depletion layer 24 is formed around the PN junction comprised of the N.sup.- -type silicon layer 14 and the P.sup.+ -type silicon area 18. As the reverse bias is increased, the depletion layer 24 connect to each other. The joined depletion layer 24 causes the reverse leakage current to decrease. As a result, the reverse-current characteristic of the JBS rectifier are similar to that of a PN junction diode, and the breakdown voltage of the JBS rectifier is improved.
If the reverse bias applied to the JBS rectifier is too low, depletion layers 24 are not connected to each other. Hence, the reverse leakage current, which flows through the Schottky barrier formed of the N.sup.- -type silicon layer 14 and the metal film 22, is not influenced. The improved reverse-current characteristic of the JBS rectifier are not achieved until the reverse bias increases such that the depletion layers 24 connect to each other.
FIG. 2 is a sectional view showing the JBS rectifier disclosed in Published Unexamined Japanese Patent Application No. 60-74582 and also in by B. J. Baliga, IEEE, Electron Device Letter, Vol. DEL-5, No. 6, 1984, pp. 194-196. As is shown in FIG. 2, wherein the same reference numerals are used to denote elements similar to those illustrated in FIG. 1, a plurality of PN junctions is formed in the surface of that portion of a semiconductor substrate 10 on which barrier metal film 2 is formed.
The semiconductor substrate 10 is made of an N.sup.+ -type silicon layer 12 and an N.sup.- -type silicon layer 14. The barrier metal film 22 is made of aluminum. Generally, it is necessary to form a thin N.sup.+ -type silicon layer in the surface of N.sup.- -type silicon layer 14 by means of ion implantation, thereby reducing the height of the Schottky barrier formed of N.sup.- -type silicon layer 14 and the metal film 22. In the JBS rectifier shown in FIG. 2, such a thin N.sup.+ -type silicon layer is not formed since the film 22 is made of metal (ex. Ti, Mo, V) which has a low Schottky-barrier property. A plurality of P.sup.+ -type silicon areas 18 are formed in the upper surface of a portion of the N.sup.- -type silicon layer 14. A guard ring 26 is formed on the layer 14, surrounding that portion of the layer 14. The guard ring 26 imparts an adequate breakdown voltage to the JBS rectifier.
When a forward bias is applied to the JBS rectifier, depletion layers 24 are formed, each being a PN junction between a P.sup.+ -type silicon areas 18 and the N.sup.- -type silicon layer 14. The depletion layers 24 are not connected to one another when a forward bias is applied to the JBS rectifier. In other words, the depletion layers 24 are spaced apart by a gap 28 as is indicated by broken lines in FIG. 2, thereby allowing the passage of a forward current. When a reverse bias is applied to the JBS rectifier, a single depletion layer 24' is formed as is indicated by the one-dot, one-dash lines. This depletion layer 24' prevents the passage of a reverse current.
The JBS rectifier shown in FIG. 2 is basically the same as that JBS rectifier shown in FIG. 1, in both structure and function, but different in that the P.sup.+ -type silicon areas 18 are spaced from one another by such narrow gaps that they are connected together to reduce the reverse leak current when the reverse bias increases over a small value, for example a few volts.
The N.sup.- -type silicon layer 14 has an impurity concentration of, for example, 10.sup.16 atoms/cm.sup.3. The P.sup.+ -type silicon areas 18 are arranged at the intervals of 6 .mu.m. Each P.sup.+ -type silicon area 18 has a square upper surface contacting the metal film 22, each side being 5 .mu.m long, and has a depth of about 2 .mu.m. When a reverse bias V.sub.R is 0 V, the depletion layers 24 have a thickness of 0.35 .mu.m, and each portion of the N.sup.- -type silicon layer 14, which is interposed between the two adjacent P.sup.+ -type silicon areas 18, has a width of 1 .mu.m.
The JBS rectifier shown in FIG. 2 is identical to the JBS rectifier shown in FIG. 1, except that more P.sup.+ -type silicon areas 18 contact the barrier metal film 22. In either JBS rectifier, the layer of the Schottky junction defined by the barrier metal film 22 and those portions of the N.sup.- -type layer 14 which contact the film 22 is less than the total surface layer of the N.sup.- -type layer 14 by the sum of the areas of the P.sup.+ -type layers 18. As has been pointed out, that portion of the N.sup.- -type silicon layer in which a P.sup.+ -type silicon area is formed has a surface area of 36 .mu.m.sup.2 (=6.times.6 .mu.m), and each P.sup.+ -type silicon area has a surface area of 25 .mu.m.sup.2 (=5.times.5 .mu.m). The surface area of each P.sup.+ -type silicon area is 30.6% of the surface area of the upper surface of that portion of the N.sup.- -type silicon layer in which a P.sup.+ -type silicon area is formed. Therefore the current efficiency of the substrate is not very high for the pellet size of the JBS rectifier.
One of the methods of increasing the current efficiency of the substrate is to reduce the surface areas of the P.sup.+ -type silicon areas. However, a reduction of the reverse breakdown voltage V.sub.B must be prevented despite the curvature of the PN junction. To maintain a sufficient breakdown voltage V.sub.B, a P.sup.+ -type impurity must be diffused into the N.sup.- -type silicon layer to a relatively large depth. When the P.sup.+ -type impurity is diffused into the N.sup.- -type silicon layer, it is also diffused in the horizontal direction, inevitably expanding the surface area of each P.sup.+ -type silicon area. The surface area of each P.sup.+ -type silicon area can not be reduced as much as is desired to increase the current efficiency of the substrate.
FIG. 3A represents the cross section of each depletion layer 24 when a reverse bias of 0 V is applied to the JBS rectifier shown in FIG. 2, and FIG. 3B illustrates the distribution of electric charge .rho., the distribution of electric field E, and the distribution of potential .phi. that are observed in the JBS rectifier when a reverse bias of 0 V is applied to the JBS rectifier. Also, FIG. 4A shows the cross section of each depletion layer 24 when a reverse bias of 3 V is applied to the JBS rectifier, and FIG. 4B illustrates the distribution of electric charge .rho., the distribution of electric field E, and the distribution of potential .phi. that are observed when a reverse bias of 3 V is applied to the JBS rectifier. In these figures, M designates the Schottky barrier metal film. Plotted on the x-axis in FIG. 3B and 4B is the depth of that portion of the substrate which is located between any two adjacent P.sup.+ -type silicon areas 18.
As is evident from FIGS. 3A and 3B, a specific charge is accumulated in the surface of the N.sup.- -type silicon layer 14 when a reverse bias V.sub.R of 0 V is applied to the JBS rectifier. As is shown in FIGS. 4A and 4B, the distribution of electric charge .rho. has a square profile when a reverse bias voltage V.sub.R of 3 V is applied to the JBS rectifier. In this case, the distribution of electric field E and the distribution of potential .phi. have similar profiles. The maximum intensity E.sub.m of the electric field and the maximum potential V.sub.d, both shown in FIG. 4B, are given: EQU E.sub.m =-(qN.sub.d W)/.epsilon..sub.s EQU V.sub.d =-(qN.sub.d W.sup.2)/(2.epsilon..sub.s)
where N.sub.d is the impurity concentration of the N.sup.- -type silicon layer, W is the width of the depletion layer, and .epsilon..sub.s is the dielectric constant of silicon.
As is understood, the profile of electric field (E) distribution has a high gradient. Therefore, the P.sup.+ -type silicon layer cannot serve to provide an adequate depletion-layer effect. The higher the breakdown voltage the JBS rectifier has, the deeper the P.sup.+ -type silicon layer must be, and the lower the current efficiency of the substrate.
As has been described, the ordinary Schottky barrier diode, whose forward voltage is low and which operates at high speed, has a low reverse breakdown voltage and a large reverse leakage current. The JBS rectifier, developed to have a high reverse breakdown voltage and a small reverse leakage current, has an N.sup.- -type silicon layer which defines a Schottky junction with the barrier metal film and whose surface area is smaller than the area of that portion of the N.sup.- -type silicon layer contacting the metal film by the sum of the areas of the P.sup.+ -type silicon layers. The current efficiency of the substrate is inevitably low because the surface area of the substrate which contacts the barrier metal film is comparatively large. The area at which each P.sup.+ -type silicon layer contacts the barrier metal film may be reduced to increase the current efficiency, but can not be reduced as much as desired because the reverse breakdown voltage of the JBS rectifier is directly proportional to said area.
The P.sup.+ -type silicon layers can be arranged at larger intervals thereby to increase the area occupied by the N.sup.- -type silicon layer. However, the larger the intervals at which the P.sup.+ -type silicon layers are arranged, the greater the reverse leakage current.